Structural design for converting a house to flats or an HMO
Structural design for converting a house or commercial building into self-contained flats or a house in multiple occupation, from new openings and stairs to fire separation floors.
About this service
Turning a London house into flats, or a shop or office into homes, changes far more than the floor plan. Floors that once carried one household now separate two, so they must resist fire and sound. New kitchens and bathrooms put heavy loads in new places, and stairs often need rebuilding to give each flat its own entrance. Our conversion package covers the structural side of all this, so Building Control has what it needs and your builder knows exactly what to build.
We mostly work for landlords, small developers and owners splitting a large Victorian or Edwardian terrace into two to four flats, or creating a licensed HMO. We also help with the upper parts of shops, where the commercial unit stays open below. If you are still deciding on a layout, our guide to converting a house into flats or an HMO explains the main structural questions before you commit to drawings.
London's older housing was never built with conversion in mind. Timber joists are often notched by generations of plumbers, chimney breasts have been removed without proper support, and rear additions sit on shallow footings in London Clay. A conversion is the right moment to find and fix these problems, rather than discovering them after tenants move in. Every set of calculations goes through our three-step quality control process, with final sign-off by our Head of Engineering.
When you need it
- You are converting a house into flats or an HMO
- You are turning offices or a shop into homes
- Building Control needs structural information for the new layout
What's included
- Survey and appraisal of the existing structure
- Design of new openings, stairs and floor strengthening
- Structural input to sound and fire separation floors
- Calculations and drawings for Building Control
How it works
- Survey the existing buildingWe visit, lift selected floorboards where agreed, and record joist sizes, spans, wall construction and past alterations. We note hidden issues such as unsupported chimney breasts or cut joists that will affect the new layout.
- Review the proposed layoutWe work through the proposed plans floor by floor, picking out new openings, relocated walls, new bathrooms and stair changes that need structural design, and flag anything that is likely to be costly.
- Design openings, stairs and floorsWe size steel beams, padstones and lintels for new openings, design trimming around new stairwells, and check whether existing joists can carry added screeds, partitions and acoustic build-ups.
- Coordinate separating floorsWorking with the fire and acoustic specialists, we confirm the structure can carry the separating floor build-up and that new steelwork has suitable fire protection specified.
- Issue to Building ControlWe issue calculations and drawings, send them to Building Control and answer their structural queries. We stay available for site questions while the conversion is being built.
What you receive
What usually catches people out on a London conversion
Separating floors are the most common surprise. A typical upgrade adds plasterboard layers, resilient bars, acoustic mat or a floating floor, and together these can add significant weight to joists that were sized for a single house. Sometimes the joists cope. Often they need doubling up, a new intermediate beam or a lighter build-up chosen with the acoustic consultant.
Stairs and entrances come next. A separate entrance to an upper flat often means a new stair through an existing floor, and sometimes a new opening in a load-bearing wall at ground level. Where that wall is shared with a neighbour, the Party Wall etc. Act 1996 may apply and notices need serving in good time. Our party wall notices service can run alongside the design.
Remember that planning, Building Regulations and HMO licensing are three separate processes. The planning team decides whether the conversion is allowed, Building Control checks the work against the regulations, and the council's housing team licenses the HMO. Many boroughs also ask for a planning fire safety statement for new homes.
Not included
- Fire strategy and acoustic design by specialists
- Planning drawings
Frequently asked questions
Do I need a structural engineer to convert a house into flats?
In most cases, yes. A conversion is a material change of use, so Building Control will check the building against the relevant parts of the Building Regulations. New openings, stair changes and floor upgrades all need calculations. Even where the layout looks simple, someone has to confirm the existing floors can carry the new separating floor build-ups. We provide that information and send it to Building Control for you.
Do separating floors between flats need to be stronger?
They need to resist fire and sound, and that usually means more weight. Extra plasterboard, acoustic insulation and floating floors all add load to existing joists. We check the joists against the proposed build-up and, where they fall short, design strengthening such as new timbers bolted alongside, a beam to reduce the span, or a lighter acoustic system agreed with your specialist.
Can I convert the space above a shop into flats?
Often yes, subject to planning. Structurally we check that the floors suit residential use, design any new openings and stairs, and look at how the flats will be separated from the shop below for fire and sound. Older parades sometimes have steel or cast iron beams over the shopfront that need assessing. Where load paths are unclear, a structural capacity appraisal is a sensible first step.
How much does a structural package for a flat conversion cost?
Our conversion to flats or HMO structural package starts from £1,200 + VAT. The final fee depends on the size of the building, the number of units and how much is changing structurally. Every job starts with a fixed fee confirmed in writing before any work begins, and our typical turnaround is 10 working days.
Will my conversion need party wall notices?
Possibly. In a terrace or semi, cutting into a party wall to bear a new beam, inserting a damp proof course or raising the wall can all bring in the Party Wall etc. Act 1996, and so can excavating for new foundations close to a neighbour. Notices must be served before the work starts, so check early. Our party wall guide explains the notice types.